My mother-in-law (90) only washes her hair once every 3-5 weeks. Is this enough?

It sounds nasty to most of the generation that grew up with daily showers but it isn't unusual for the elderly (women) to go weeks or months without their hair being washed.
Bed bugs are an issue for anyone in contact with her.Ridding her house of them shouldn't be up to her at that point.Bed bugs spread like fleas.It's a sanition issue for all concerned.
I'm constantly amazed at the many caregivers here who can clearly see their loved one's judgement is impaired yet they try to reason with them.
I stopped asking mother's premission to do things for her once her decisions became clouded.
I'd tell her the family has made the decision in her behalf and if she doesn't like it (which she won't) you will call the authorities.
She will hate you but I doubt she wantsher "dirty secret" to become public knowledge.

vaccuming dont do s*it for bed bugs pam . they hang out in crevaces that a vaccumn wont reach and even if you utilized vacc'n youd need to suck up a handfull of DE in the vacumn after every use to mess up the bugs youve sucked up . vaccumning aint your friend with bedbugs . if you put down the DE hard , then proceed to vaccumn it back up , your pissin up a roap .

Would she let you cut her hair?...I clip my moms hair almost like a mans crewcut...very easy to care of and wash even if you don't wash that frequent

KaarenM, we found out that mom is less functional early in the morning, so we got an 8AM appointment with her MD. Her speech was garbled, her thoughts were jumbled and he finally saw the problem. Whatever works, do it.

She went to her doctor yesterday. He brushed it off and was not helpful. She can pull it together for her ten minutes him. Oh well.

Will she go to her doctor? And listen to him/her? Make an appointment, and give the doc a preview, asking specifically that they write a prescription for shampoo 1-2x/week. They can even specificy Neutrogena Pine Tar shampoo or something else.....the idea is to have the doctor write it down as a required treatment (which will never end).

I had a patient who hadn't had a hair washing in 8 months.

I agree with Sophe509: Ick!!

Personally, I'd get professional help for her. Starting with a professional exterminator, housekeepers and a geriatric case manager. I would not visit in person so long as she has bedbugs. Ick. Hair should be washed weekly, esp with bedbugs in the equation. Nor let any of your immediate family visit her in person either. Don't want those things in your house.

Considering bedbug eggs hatch in about 6 days, that's the longest I would let her go without a thorough bath and shampoo. Wash all bedding, that includes pillows and blankets. Wash clothes including coats and hats. Vacuum daily including close to baseboards and any crevice you find.
I wouldn't want her in my car or house either.